A slide-fastener stringer may comprise a pair of slide-fastener stringer halves each of which can include a fabric support tape or band and a coupling element affixed to an edge of the support tape and adapted to engage or disengage with a complementary coupling element of the other support tape. The coupling elements may be more or less identical and can have a plurality of coupling members relatively closely spaced apart along the edge of the tape and generally projecting laterally therebeyond.
Each coupling member may comprise a coupling head having formations projecting in the longitudinal direction, i.e. parallel to the edge of the tape along which they are disposed, fastening portions or shanks extending from each head transversely to this edge to overlie the tape and provide a means whereby the coupling element or member may be affixed to the tape and, in the case of continuous coupling elements, a connecting portion or ligature joining the coupling members together inwardly of the tape edge.
The coupling heads of the two coupling elements interdigitate, i.e. fit behind one another, so that the aforementioned formations engage to prevent or restrict lateral separation of the coupling elements in the absence of movement of a slider along the two coupling elements. The slider functions to bring the two coupling elements together or to slide them apart in effecting the coupling or decoupling movement.
As pointed out in the aforementioned copending application, various techniques have been proposed heretofore for securing the coupling elements to the support tapes. These techniques may generally be considered to lie in three categories. In a first category is the attachment of the coupling element by loops of thread or by stitching the coupling element to the support tape with a row of chain or lock stitches, or by fitting the coupling heads through openings in the tape or by weaving or knitting the coupling elements into the tape. A second technique is the clamping of the individual heads to edge of the tape by crimping or like technique. This latter method is used most frequently where the coupling element consists of a discontinuous chain of individual coupling members. In the third technique, continuous coupling elements with a row of closely spaced heads are thermally welded or adhesively bonded to the support tape or are anchored thereto by rivets, studs, or like widely-spaced pin-like formations inserted into or formed on the extruded coupling element.
Several disadvantages have been encountered with the prior-art use of the last mentioned technique for securing coupling elements to a tape. Where the adhesive bonding or thermal welding approach has been used heretofore, it has been the customary practice to bond the entire surface of a coupling element confronting the tape to the latter. This greatly limits the flexibility of the coupling element and prevents the technique from being used effectively for fine coupling elements, i.e. coupling elements with a very close spacing of the coupling heads.
When pins, studs or rivets have been used as described, they penetrate the fabric and are formed with heads on the side of the tape opposite that from which they extend. The difficulty with this system is that considerable localized stress is applied where each stud engages the fabric and these stresses can be sufficient to break the stud or tear the fabric, thereby releasing the coupling element and rendering the stringer inoperative.
In the production of continuous coupling elements from thermoplastic synthetic resins, it has been proposed heretofore by myself and my coworkers on behalf of the present assignee, to pass the support tape through an extrusion head and to extrude a synthetic resin strand onto and around the edge of the support tape, the strand being shaped by a wheel provided with forming cavities or recesses along its periphery. It has also been proposed to continuously extrude a coupling element of thermoplastic synthetic resin by passing an endless mold member provided with a row of cavities through an extrusion head which fills these cavities and then to strip the resulting shaped strands from the endless mold thereby provided. In the first instance, the bond between the coupling element and the tape is effected by imbedding the tape in the coupling element strant with the disadvantage that shrinkage of the system occurs with a variation in the spacing and orientation of the coupling heads which cannot be tolerated when the coupling elements are to be provided on garments. In addition, the penetration of the synthetic resin varies from place to place along the textile tape and thus the quantity of synthetic resin available to fill each mold cavity may vary so that some heads are incompletely formed. In general neither of the aforementioned techniques, as practiced heretofore, is capable of the necessary precision to make fine slide fastener stringers at high rates and with good mechanical properties, i.e. high break or release resistance.
I have found, more specifically, that where the thermoplastic synthetic resin penetrates deeply into the body of the tape, the thermoplastic mass along the edge of the tape stiffens the latter while the tape fibers stiffen the thermoplastic mass so that a major part of the flexibility, desirable in a slide-fastener stringer, can be lost. Stiff slide-fastener stringers cannot be used in the garment industry.
Where attempts have been made to form the coupling element continuously and mount it upon the support tape by welding by thermal bonding techniques, the aforementioned problem has not fully been eliminated since the substantially uniform bonding of all juxtaposed surfaces of the tape and the coupling elements also brings about stiffening to an undesired degree.